Fimbul is the linchpin of Varuna grids, with most wanting at least two and late-game setups often using three. As the shop has seven copies, you need one drop to finish two weapons. While an acceptable filler in Leviathan Omega grids, it isn't generally worth the resource investment to farm, especially for new players

Note that because this event has historically re-run very infrequently, it is strongly recommended to farm two 4★ Fimbuls for any player who may wish to build a Varuna grid within the next sixth months

February 2018 Iteration: Cygames has announced that players will be able to acquire a primal-like summon for free during the upcoming "What Makes the Sky Blue Part II" event in Late February, which will open primal grids (including Varuna) to many players. Acquiring Fimbuls should be a high priority if you are planning to choose said summon

All that said, Gleipnir is still a respectable filler, especially for single-primal setups such as Europa+Varuna, where it may be more difficult to reach the Normal-skill DA cap with only a single primal aura

Fenrir has both a lackluster aura and call, but consider getting her if you own and use Bonito, as Fenrir is an acceptable Water summon filler due to her base ATK/HP from having a final uncap

For players interested in all three rewards: many copies of Fenrir and Gleipnir will likely drop while collecting treasure for Fimbuls, so it is not recommended to purchase the former from the shop until the end of the event

Most materials should come naturally while farming Crescent Cuff, where players can elect to either:

Farm NM120 for more cuffs per Nightmare proc to acquire the shop copies more quickly. This is recommended for players who can clear NM120 (or 100) reasonably easily, want to conserve Half Elixirs, and are only seeking two Fimbuls

skills, White Rabbit, or Kaguya, increasing the likelihood of additional Fimbul drops, potentially reducing the number of shop copies which much be purchased. This is recommended for players for whom real-time/resource consumption is not a concern, especially those seeking three Fimbuls. Naturally, one can always switch back to NM120 to focus on the Shop stock once enough drops have been acquired